1. Field
The field of the invention relates to particle beam writing and lithograph technologies for fine image fabrication and, in particular, to proximity effect correction.
2. Description of Related Art
For generating fine images on a plate by lithographic technologies, particle beam writing methods and optical projection methods that uses a photo-mask consisting of transparent and opaque parts have been used. An example of particle beam writing technology includes an electron beam writer, which is used for writing fine images on both silicon wafers and glass masks for optical projection lithography. A technology that uses electron beam writers for writing fine images for semiconductor integrated circuits directly on the semiconductor wafer is referred to as electron beam direct writing (EBDW) technology.
A fundamental problem with conventional lithographic technologies is image quality degradation and resolution limits caused by chemical and physical effects in the process of the technologies. The degradation or difference between obtained image and intended pattern worsens when image size is increasingly reduced. Proximity effect is a dominant issue among these effects.
Proximity effect correction is mandatory for a particle beam writer, such as an electron beam writer. Proximity effect is a degradation or variation of a written image caused by scattering of incidence particles in resist or backscattering or reflecting particles from lower layers of the resist. In general, the plate or photomask is coated with particle beam sensitive resist.
Conventional electron beam (EB) writers can write complicated patterns by one shot. Variable Shaped Beam (VSB) type EB writers can write a predefined rectangle/triangle/trapezoid pattern by one shot. Cell Projection (CP) type EB writers can write complex patterns defined on a stencil plate by one shot. However, the size of CP cells on the stencil plate has limitations. The allowable size of the commonly used CP cell is several microns by several microns.
In both VSB and CP type writing, the dose or number of particles, such as electrons, in a shot can be controlled by exposure time. In one aspect, the quantity of injected particles is referred to as the dose. FIG. 1A (A) 100 shows a low dose case, and FIG. 1A (B) 102 shows a high dose case. Referring to FIG. 1A, a higher dose 110 provides more fat images 120 as a result. In one aspect, “Threshold” indicates a level of dose intensity over which an image of the lithography appears.
Embodiments of several proximity effect correction methods are proposed herein. One embodiment refers to a dose correction method that corrects proximity effect by controlling dose used for VSB or CP shots. This is a correction method that uses pattern fattening phenomenon caused by increasing dose, as shown in FIG. 1A. Another embodiment refers to a proximity effect correction (PEC) by pattern that corrects proximity effect by modifying writing patterns or adding auxiliary patterns. These auxiliary patterns are sometimes referred to as assist pattern or dummy pattern. PEC methods, that use pattern modification like slimming down or fattening up some parts of the pattern and add serif to the pattern for obtaining intended images, are also classified as a PEC method that uses auxiliary patterns. Hereafter, all these types of correction methods may be referred to as Proximity Effect Correction (PEC) by pattern.
Most of the EB writers for mask writing are realized only by VSB type functionality. PEC by pattern method is not suitable for VSB type EB writers because pattern modification or auxiliary patterns increase the number of shots needed, and as a result, writing time becomes longer. Therefore, the dose correction method is dominant for VSB type EB writers. On the other hand, by using CP cell type writer, plural patterns are written by one CP shot and dose correction among patterns in the CP cell or to use different dose for writing patterns in the CP cell is impossible. As a result of above mentioned discussion, a problem occurs in which the width of obtained images vary depending on the position in the CP cell, although the target width is identical among those patterns.
In general, there is no way to correct proximity effect by dose modulation between patterns in a CP cell because those are written by one shot. One of the few possible methods is to correct the proximity effect by pattern modification. When use of Electron Beam Direct Writing (EBDW) is considered for wafer writing, maximization of writing by the CP cell method is preferable for shortening writing time. However, there currently exists a need for a proximity effect correction method that is applicable to both CP and VSB writing and satisfies the demand for speed and accuracy. Even for mask writers, the use of CP cells contributes to the improvement of writing speed.
A proximity correction method that aims for high accuracy by correcting both dose and pattern and is applicable to both VSB and CP writing has been proposed. However, the proposed PEC by pattern focuses on only one direction of the pattern consisting of line width and does not consider line edge shortening. There currently exists a need for a more accurate PEC that considers not only one-dimensional correction but also two dimensional corrections.
Conventional technologies for proximity effect correction by dose or dose correction has been proposed. However, proximity effect correction by conventional dose correction methods is not enough. Proximity effect correction without modification of the writing object pattern has limitations, and conventional methods have no means to modify patterns.
FIG. 1B depicts a conventional issue caused by the mixture of writing by VSB (Variable Shaped Beam) and CP (Cell Projection). A pattern indicated by CP Cell-I is shown in this example. The CP cell comprises three rectangles, R1, R2 and R3. Deposited energy by nearby patterns on a line AA′ is depicted by a curve indicated by “Deposited Energy” in chart 130. In chart 130, a left vertical axis shows “Deposited Energy” by incidence of the particle beam including backscattering from the neighborhood, and a vertical axis in right side shows dose intensity for each writing object. A horizontal axis shows positional coordinate on the line AA′. For CP cell writing case, all patterns included in a CP cell, for this specific case, R1, R2 and R3 are written by the same dose intensity. R1, R2 and R3 are written by the same dose indicated by “Dose for CP”.
FIG. 1B illustrates the limitation of conventional dose correction that is exposed by the structure of writing objects. As shown, a case of writing a CP Cell, indicated by CP Cell-1, is considered. Writing objects located near CP Cell-1 create a distribution of deposited energy indicated by “Deposited Energy” by backscattering. Optimal dose intensity for each rectangle R1, R2, and R3 are independently determined as indicated by “Dose for R1”, “Dose for R2”, and “Dose for R3” in the distribution of deposited energy. However, pattern dimension differences occur between R1, R2, and R3, if a CP Cell is used and as a result same dose intensity is applied to three writing objects. Even VSB writing and enough large writing pattern are assumed, the case could occur in which correction by dose cannot compensate the proximity effect.
FIG. 2 shows an example of conventional dose correction results for a mixture of CP cell and VSB writing objects. The dose intensity of each writing object is indicated by a number. Rectangles indicated by V00 thought V25 are writing objects written by VSB. A number, such as 0.666 in each rectangle, is dose intensity calculated by a correction program for the writing object. This pattern is an example of a situation where relatively small CP cells are placed on a wide large rectangle consisting of V00˜V25.
FIG. 2 depicts a layout example that may cause the above mentioned problems. In this layout, backscattering by large rectangle pattern written by VSB shots V00˜V25, provides a large influence on patterns written by CP Cells indicated by C00˜C25. Especially, influence to CP Cell, C00˜C05 is large. As a result of the deposited energy gradient, dose for C00˜C05 is different from that for other CP Cells. In the example of FIG. 2, the size of the CP Cell is 2.15 um in width and 3 um in height, and the distance between the bottom of the CP Cell and the top of VSB is 1.25 um.
The problem discussed above can be solved by changing patterns written by the CP Cell to VSB and by fracturing patterns small enough to avoid the problem. However, changing the CP Cell to VSB and miniaturization of the writing pattern causes a problem of a long writing time. There currently exists a need to solve inconsistent demands for accuracy and short computation time.
One of the possible solutions for the above mentioned issues is an adoption of “Correct-by-construction” approach that is characterized by following the steps of: finding a temporary solution of dose correction by conventional method and improving the accuracy of the solution at the region. “Correct-by-construction” approach is not reported in the dose correction area. The reasons for this include a lack of technology that detects inaccurate part of a whole chip layout from dose correction point of view and effective and practical way of improving the accuracy of dose correction. There currently exists a need to provide both accuracy of writing and high throughput to particle beam writers.
In LSI design process, partial modification of the layout by design change occurs frequently. Thus, there currently exists a need to provide an efficient recalculation method of dose correction.